Advanced ceramics such as ytterbium-doped yttria (Yb:Y2O3) are excellent laser media due to their hardness, strength, and transparency in the range of 0.4 to 10 μm. Their thermal properties enable the laser to operate at higher temperatures and dissipate heat generated during laser operation better than other laser materials, such as yttrium aluminum garnet (YAG).
However, single crystal Yb:Y2O3 is difficult to produce in the large sizes and necessary configurations for high-power lasers. Polycrystalline Yb:Y2O3 can be used in such high-performance applications if it is produced in a manner as to produce a fine grained transparent material with clean grain boundaries, very low porosity, and less than 10 ppm levels of impurities.
Transparent polycrystalline Yb:Y2O3 laser material conventionally has been produced by sintering ceramic powders in a process whereby a nanosized ceramic powder is cold-pressed into a green body having the desired shape which is then heated without pressure to form the final product. This process is different from sintering by hot-pressing, in which the ceramic powder is heated under pressure to form the final product. Although the powder being hot-pressed must not melt to a great extent, some melting of secondary phases in the powder or surface melting can be allowed, and in the case of porcelains and clay products, the melting of these secondary phases can act as an intrinsic sintering aid to “glue” the primary solid particles together with a glassy phase to form the solid ceramic material.
However, advanced ceramics do not have these intrinsic sintering aids and therefore some extrinsic sintering aid must be added. For example, materials such as Yb:Y2O3 often are mixed with a secondary material such as lithium fluoride (LiF) as a sintering aid. Some sintering aids may liquefy at or somewhat below the primary material's densification temperature thereby promoting liquid phase sintering. Other sintering aid materials exhibit higher solid-state diffusion coefficients than the primary material's self-diffusion coefficient. The secondary material may conversely have a lower solid-state diffusion coefficient that prevents exaggerated grain growth and promotes grain boundary refinement and pinning. The sintering aid may also simply clean or etch the primary material's surfaces thereby enhancing solid-state diffusion.
For small batches, the powdered sintering aids can be mixed with the powder to be sintered with a mortar and pestle. In larger batches, mixing can be accomplished by ball milling, attritor milling, high shear wet milling, and variations or combinations of these methods. However, in the case of optical or laser quality polycrystalline materials, homogeneity must be measured on the nanometer scale, and mechanical mixing results in homogeneity of the sintering aid that is only in the hundreds of microns range, a level of homogeneity that is several orders of magnitude too high to produce optical or laser quality polycrystalline material. Inhomogeneity of the sintering aid within the ceramic powder results in areas that have too much sintering aid and other areas that have little or no sintering aid. While this is generally not too important in fabrication of materials that are relatively easy to sinter or in opaque materials, it is a major problem in the fabrication of transparent ceramics.
The sintering aid must also be removed from the densified material to prevent it from being trapped and forming pores in the material. In the case of optical ceramics, if the sintering aid is not effectively removed, the pores formed by the trapped material can cause high scattering and absorption losses in the final article. The scattering sites in such ceramic materials are typified by inclusions and voids that appear white when viewed in reflected light. The absorbing regions are dark when viewed in both transmitted and reflected light. In such a case, the article does not possess uniform optical losses, and consequently the yield is poor, costs are high and large size and different shapes are not possible to manufacture.
In the case of Y2O3 ceramics using LiF as a sintering aid, the inclusions contributing to optical scattering are due to trapped LiF that was not removed during sintering and by compounds that resulted from impurities in the starting ceramic powder and sintering aid. Additional scattering is caused by the presence of voids, i.e., pores, that possess very high scattering efficiencies. The absorption losses are caused by oxygen vacancies which arise from the presence of carbon related impurities. The carbon-related impurities react with oxygen in the Y2O3 to produce CO/CO2 volatile gases and leave behind vacancies in the molecular structure, causing the material to turn black. The extent of the absorbing regions is influenced by the LiF sintering aid, which reacts with the carbon to form volatile carbon-fluorine species that can be easily removed. Not enough LiF leads to oxygen vacancies, whereas excess LiF gets trapped and leads to optical scattering.
Transmission losses in such transparent ceramics are also due to absorption caused by impurities including transition metals and other ionic species present in the ceramic. Scattering in transparent ceramics can be minimized by optimizing the processing conditions such as powder size distribution control, sintering pressure, and sintering temperature. However, absorption loss caused by the presence of various transition metals and other ionic impurities in the ceramic is critical since such defects lower the transmittance and have a detrimental effect on lasing.
Sintered Yb doped Y2O3 ceramic materials have had a demonstrated use as laser materials. See A. A. Kaminskii et al. “Lasing and Refractive Indices of Cubic Yttrium Oxide Y2O3 Doped with Nd3+ and Yb3+ Ions.” Crystallography Reports, Vol. 48, No. 6 (2003) pp 1041-1043; and J. Kong et al. “9.2 W diode-end-pumped Yb:Y2O3 ceramic laser.” Applied Physics Letters, Vol. 86 (2005) pp. 161116-1-161116-3. Sintering is achieved using nano-sized starting powder that is uniaxially cold pressed and then cold isostatically pressed into a green body, which is then heated in vacuum without external pressure or load. The nanopowder is a strict requirement and highlighted by previous authors in their patents. See U.S. Pat. No. 6,825,144 to Hideki (polycrystalline laser gain media are limited to crystals having a mean grain size of less than 20 μm, and laser quality ceramic cannot be made if the grains are larger).
Nano-sized powder has a huge driving force to lower the surface energy and so heating at elevated temperatures allows densification (sintering) to take place whereby the grains grow slightly and the surface energy is lowered. Typically, the grains are a few microns in size. However, the grain size must remain small so that pores can be removed to enable production of laser quality ceramics. If prolonged times or higher temperatures are used in an attempt to eliminate porosity, the grains grow larger than about 25 μm, and it becomes difficult to remove the pores. In addition, in such cases, significant grain growth will occur, which makes it even more difficult to remove the pores.
It has been assumed that it is not possible to make a laser quality ceramic from Yb doped Y2O3 using hot pressing. In hot pressing, a sintering aid is mixed with the powder. The mixed powder is placed in a hot press, evacuated, and then heated up to the densification temperature, at which time a load of several thousand psi is applied. The problem has been that the sintering aid, which prevents carbon contamination from the hot press die, typically leads to grain growth. In fact grains are typically larger than 30 μm and more typically larger than 100 μm. Therefore it was widely believed that hot pressed Yb doped Y2O3 will not produce laser quality ceramic since the porosity cannot be reduced. In fact this rationale has led the other research groups away from using hot pressing.
If one could solve the problem of scattering using hot pressing, then not only could laser quality ceramics be attainable, but other advantages of hot pressing could be exploited. These include scalability to large sizes and complex shapes, relatively short processing times, and mass production. Manufacturing costs are lowered and the potential of making high power lasers, including >>KW class lasers, becomes more viable.